1. Field of the Invention
The present invention relates to ignition controllers for gas-fired forced combustion air heating appliances, and more particularly to a relay sequencing apparatus and method for controlling a multiple tap induction motor to provide different motor speeds.
2. Description of the Prior Art.
With past conventional controls of fractional horsepower induction or permanent-split-capacitance motors, typically used in residential circulating fan appliances, the starting or speed changing of these motors is done without any intelligent control. When starting the motor winding taps or changing between motor winding taps to change motor speed, high load currents pass through the mechanical relay contacts that perform the switching. In order to randomly change motor speed at any time, the relay corresponding to the desired motor speed must be powered directly. Due to the lack of intelligent control, all the relay contacts must be capable of making or breaking the motor load current. The relay contact requirements in a typical central heating furnace eliminate many lower cost relays from consideration because all the relay contacts must make or break the load current for a large number of switching cycles.
The major drawback in using lower cost relays is that they have a much lower current switching capacity on their normally closed contacts than on their normally open contacts. These lower cost relays can carry high load currents with the normally closed contacts, but if they must make or break the current with the normally closed contacts they tend to have premature contact welding failures. These failures Occur because the normally closed contacts must only rely on magnetic forces to overcome both mechanical spring forces (which bias the contacts toward a normally closed state) and welding forces (which occur as the contacts begin to open) in order to open the contacts.